ABSTRACT Although CO2 is prominent as the most important greenhouse gas, respon-sible for 64% of anthropogenic global warming, it is also a viable indicator for indoor air quality (IAQ). Due to the incessant increase in the human population and residence time indoors, the need to control indoor CO2 levels has become exigent. To this effect, dry-phase removal technology via adsorption with zeolites, activated carbons (AC) and activa-ted carbon fibers (ACFs) had sufficed. Chemically modified AC and ACF surfaces through alkali impregnation have been used to improve their selectivity toward CO2 at room tem-perature. Here we appraise the various methods in the literature and carry out perfor-mance evaluation based on the physical and chemical modification induced by the chemi-cal agents and experimental conditions. This study reviews the improved adsorption of low concentration (0.3%) via surface reformation of commercial carbon-based adsorbents, and the highest adsorption capacity was 2.2 mmol/g CO2 at the indoor level, which was achie-ved by AC pellets doped with ammine functionalities.

KEY WORDS Indoor Air Quality, CO2 adsorption, Activated carbon, Activated carbon fiber, Surface Chemistry

1. INTRODUCTION

Aside from being the main anthropogenic contributor to global warming, CO2, an index indicating indoor air quality (IAQ), is harmful at relatively high concentra-tions, especially in confined spaces such as offices, schools, subway stations, cars, airplanes, and submarines. Indoor CO2 concentration (IAQCO2) above 1000 ppm, causes unpleasantness, fatigue and headache. Therefore, effective management of IAQCO2 is necessary for human health (Satish et al., 2012).

According to Park et al. (2020), IAQCO2 in classrooms usually falls in the range of 1,000 to 3,000 ppm. Elsewhere, office meeting rooms have exhibit between 446 and 1,450 ppm CO2 with normal ventilation, which could rise to 4,920 ppm without ventilation (Vehviläinen et al., 2016). When uncrowded, the IAQCO2 of subway cabins ranges from 550 to 2,680 ppm (Lee et al., 2014). However, it often exceeds 5,000 ppm during rush-hours. Due to the commonly elevated IAQCO2 in public indoor spaces, the need for its monitoring and control. As set by the Environmental

Dry Capture of Low-level CO2 from Public Indoor Spaces using Chemically Modified Carbonaceous Adsorbents - A Review

Sujeong Heo, Wooram Kim, Tae Jung Lee, Adedeji A. Adelodun1), Young Min Jo*

Department of Applied Environmental Science, Kyung Hee University, Yogin-si, Gyeonggi-do, Republic of Korea 1)Department of Marine Science & Technology, School of Earth & Mineral Sciences, The Federal University of Technology, P.M.B. 704, Akure, Nigeria

*Corresponding author. Tel: +82-31-201-2485 E-mail: ymjo@khu.ac.kr

Received: 14 December 2020 Revised: 26 January 2021 Accepted: 1 February 2021

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Vol. 15, No. 1, 2020132, March 2021doi: https://doi.org/10.5572/ajae.2020.132

ISSN (Online) 2287-1160, ISSN (Print) 1976-6912

Review Article

Copyright © 2021 by Asian Association for Atmospheric EnvironmentThis is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Open Access

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Protection Agency (EPA), IAQCO2>1,000 ppm requires efficient control technologies to ensure the safety of humans during their regular long-duration operations in confined spaces (Vehviläinen et al., 2016).

Among the various techniques for CO2 capture, dry-phase adsorption is the only process feasibly applicable to indoor scenarios. Considering the low-concentrations of IAQCO2, natural (inevitable) and artificial (necessary) sources, incessant generation, and evasiveness, an adsor-bent with a high affinity for CO2 molecules, hydropho-bicity, sanitary safety, ready availability, mechanical stabil-ity and easy regeneration is required for sustainable use. Such an adsorbent should be efficient enough to operate under ambient conditions.

Thus, this study comprehensively explores plausible techniques to capture CO2 at low levels of 3,000 ppm, as studied in some references ( Jeong et al., 2019; Kim et al., 2017; Adelodun and Jo, 2013), focusing more on dry adsorption. In this review, ultra-high-efficiency materials such as metal organic frames (MOF), carbon nanotubes

(CNT) and hierarchically porous zeolites including ZSM and SBA were not addressed due to their high cost and lack of feasibility for indoor fields.

2. ADSORPTION OF IAQCO2

In the past two decades, researchers have intensified efforts to develop adsorption technology to control of IAQCO2 at room temperature. Popular among the appro-a ches is the use of amine-based or amine-functionalized adsorbents, derived by tethering amine groups onto sup-ports by grafting or chemical impregnation and immobili-zation of liquid amines (Moloney et al., 2011).

When compared to other technologies, adsorption offers some distinct advantages, particularly for indoor air control through product recovery, non-necessity of pollutant control during product recycling, automation, selectivity at trace and ultra-trace levels, excellent control and response to process change, cost-effectiveness, envi-ronmentally friendliness, high adsorbent reusability cycles, high energy efficiency etc.

The major factors that influence adsorption include the chemical properties of the chemical properties of the adsorbate and adsorbent, the adsorbent’s surface area and porosity, experimental conditions such as temperature, humidity, flow rate, gas matrix, pressure, etc.

Solid adsorbents (in packed or fluidized beds) are more energy efficient than wet scrubbers, in which CO2 molecules dissolve into the fluid. Whereas in adsorption, the process involves either weak physisorption by van der Waals attractions or chemisorption by covalent bonding interactions between the CO2 molecules (adsorbate) and an adsorbent surface. However, no sharp distinction exists between physical and chemical adsorptions. Rather a somewhat gradual transition scaled by the strength of the interaction at the interface occurs. At the equilibrium point at which adsorption rate equals that of desorption, the CO2-laden solid can be refreshed by subjecting it to one of pressure, vacuum, or temperature swing adsorp-tions, i.e. PSA, VSA and TSA respectively. Recently, com-binations of two or more of these techniques have been adopted to achieve faster and more efficient regenera-tion.

Zeolites, composed of Al and Si oxides, are an easily-attainable, commercially-available adsorbent with a high adsorption affinity for various gases. Zeolites are micro-porous crystalline materials with adsorptive and ion exchange properties, often used to capture gaseous pollu-tants due to their characteristic high surface area and three-dimensional pore structures (Ye et al., 2012; Liu et al., 2011). The type of surface cations, polarity, surface area, and pore volume determine zeolites’ adsorption capacities (Plaza et al., 2007).

Amongst the noble functional zeolitic crystals, ZSM structures are excellent ones purposefully designed for CO2 capture. These zeolites tend to maximize electrosta-tic interactions between the surface and CO2 molecules. Besides, metal organic framework (MOF) is ano ther porous solid material of immense porosity up to the region of 10,000 m2/g (Ghanbari et al., 2020). These three-dimensional adsorbents consist of secondary build-ing units and connectors, whose length can be adjusted depending on the target molecule’s kinetic diameter. However, the cost of gaseous adsorption using these ‘future’ materials is still high, and it also requires handling expertise ( Jiao et al., 2016; Wang et al., 2014).

On the other hand, carbon-based adsorbents such as activated carbon (AC) and activated carbon fiber (ACF) are incredibly porous materials with appreciably high surface area to unit weight ratio. These carbon-based adsorbents are currently the most popularly used due to their ruggedness, robustness, relatively low-cost, and reusa bility (Yu and Chuang, 2017). Capturing indoor air pollutants using carbonaceous adsorbents at normal

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temperatures is considered sustainable. Thus, recent adv-an ces on the surface modification of carbon-based adsor-bents to enhance their selective and capacitive IAQCO2 adsorption are appraised in this study.

Adsorption selectivity is the measure of the preferen-tial attraction of a material towards one or a few compo-nents in a mixture or matrix in which efficient contact is feasible between the target and the material (Adelodun et al., 2015).

Selectivity in a physical adsorption system may depend on differences in either equilibrium or kinetics, but the great majority of adsorption separation depends on equi-librium-based selectivity (Clarkson and Bustin, 2000). The selective adsorption of CO2 has become a tremen-dous research interest in recent years. Most of these stud-ies on the selective separation of CO2 from N2 atmo-spheres include the use of metal-organic frameworks

(Wu et al., 2014; Chen et al., 2013; Hou et al., 2013), syn-thetic zeolites (Sethia et al., 2014; Dangi et al., 2012), polymers (Nagarkar et al., 2011) and carbon-based adsor- bents (Chandra et al., 2012; Thote et al., 2010; Plaza et al., 2009a).

When properly modeled, the experimental investiga-tion for the selective capture of IAQCO2 proved that the matrix of the inlet gases should comprise other common air constituents, be they natural such as Ar and moisture or anthropogenic such as VOCs and particulate matters. Thus, with CO2 being the major weak Lewis acid in indoor air, its selective adsorption is feasible by increas-ing the surface basicity of the adsorbent (Adelodun et al., 2015).

3. ACTIVATED CARBON (AC)

Due to the established demerit of zeolites with CO2 adsorption at high humidity, activated carbons are suffi-cient as suitable alternatives. ACs do not require any moisture removal, and they present a high adsorption capacity at ambient pressure. Moreover, they can easily regenerate for multiple reuses with no or insignificant drop in original efficiency (Plaza et al., 2009b). The sur-face chemistry of AC can strongly affect the adsorption of CO2

(Pevida et al., 2008). Due to the acidic nature of CO2 with weak Lewis acid, it is expected that the intro-duction of Lewis bases onto the AC surfaces may incre ase the adsorbent’s affinity toward CO2 molecules. Besi d es, surface impregnation, basifying the AC surfaces also

includes increasing the π-electron density of the basal planes, which has also been reported to exhibit basic properties (Lahaye, 1998). To achieve surface basifica-tion of AC, heat treatment and amine-doping such as amination and ammoxidation are the respective physical and chemical approaches popularly used (Dali et al., 2012). In addition to this, the CO2 capture capacity could be enhanced by engineering the porous structure of AC in a hierarchical design adapted for the adsorbate. Sevilla and Fuertes (2011) and Sevilla et al. (2011) reported that ACs predominated with 1-2 nm width pores often exhibit excellent CO2 adsorption selectivity, unlike those with narrower micropores or with a mesoporous pore network.

Elsewhere, some efforts had been exemplified to use AC to support the highly successful CO2 absorbents by implying monoethanolamine (MEA) and glycine (Gly). This approach was conceived to improve the basic adsor-bents’ portability, a property imperative for indoor use

(Lee et al., 2013). Later, chemical modification by alkali-metal ion exchange also was carried out on Gly-based absorbents (Lim et al., 2016). However, such alkali impregnation significantly reduce the adsorbent’s specif-ic surface area and microporosity. Such depreciation in the textural properties resulted in a decrease in pure CO2 sorption implying low CO2 storage capability. Contrarily, the 5 M supported MEA exhibited significantly enhan-ced low-level CO2 capture capacity from 0.016 to 1.064

mmol/g consistently. The surface of the chemically modified AC adsorbents

as observed by X-ray photoelectron spectroscopy (XPS) was tethered with amine, pyrrole and quaternary-N. In reducing potency, pyridine, amine, and pyrrolic-N have been identified as N-functionalities that contribute sig-nificantly to CO2 selective adsorption (Adelodun et al., 2015; Lim et al., 2015).

3. 1 Amination of ACA highly bilateral interforce between adsorbent and

CO2 molecules could be formed by providing strong covalent bonds with metal oxides as those found in MOFs. However, the financial cost to fabricate, handle, and regenerate MOFs are excessively high (DeSantis et al., 2017). For this reason, efforts have been made to up the selective efficiency of AC, being a cheaper and readily available alternative. In accordance, amine groups are on the AC surface using amination, a process in which ther-mally fragmentized ammonia radicals are impregnated

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on AC surfaces. The process is theoretically depicted in Fig. 1.

At temperature between 200 and 1, 200°C, the amine radicals viz. .NH2, :NH, atomic :N and .H, would attack the carbon surface to form surface nitrogen functional-ities (SNFs) (Shafeeyan et al., 2010; Przepiorski et al., 2004; Soto-Garrido et al., 2003). Amination simply basi-fies the AC surface, turning it into a Lewis base-laden environment onto which CO2, a Lewis acid, could be eas-ily scavenged. The pioneering studies on amination in the past decade are summarized in Table 1. Here, we found the stock material for the AC to vary widely.

3. 2 Ammoxidation of ACInstead of pure ammonia gas used in amination, ammo -

xidation involves using ammonia-air gas mixture for nitro-gen enrichment of carbon surface. Jansen and Bekkum

(1994) observed differences in the surface chemistry of

aminated and ammoxidized carbons carried out at 600-900°C. Therein, they reported differences in the amide-lactam-imide ratio between the techniques. The respec-tive nitrogen contents and surface fractions obtained were 4.4 wt% and 0.14 wt% respectively for amination, while ammoxidation achieved 3.2 wt% and 0.11 wt%.

According to Pietrzak et al. (2007), ammoxidation with ammonia: air of 1 : 3 at 300°C, depends mainly on the period of the process ammoxidation is performed. They reported that the ammoxidation of brown coal decreases its surface area. Whereas, when used as a thermal activa-tion process, it favors the formation of surface oxygen groups (SOFs) as well as enhancing the carbon’s basicity. However, using the similar ammoxidation condition

(NH3 : air = 1 : 3) but at 350°C, a further study carried out by Pietrzak (2009) on volatile bituminous coal resul-ted in enhancement of both the textural (pore structure and surface area) and surface basic property as varieties

Fig. 1. Schematic diagram showing the amination process (Kim et al., 2017).

Table 1. Summary of surface amination of AC adsorbents for CO2 capture.

Material Treatment Target Reference

Almond shell 800°C; 2 h CO2 Plaza et al., 2009Commercial 200-1000°C; 2 h CO2 Przepiorski et al., 2004Wood & peat 200-800°C; 2 h CO2 Pevida et al., 2008Olive stone 400-900°C; 2 h CO2 Plaza et al., 2009Biomass, sludge, coke 400°C; 2 h CO2 Gray et al., 2004

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and large amount of SNFs were incorporated.The thermogravimetric (TG-DTG) curves assessed by

Wachowski and Hofman (2006) showed that coal sam-ples ammoxidized at higher temperature exhibit slightly lower thermal stability. This occurrence could be due to the formation of less refractory SOFs usually associated with air-treatment at low temperatures.

A group of researchers investigated and identified a peculiar application of ammoxidized carbons. They rep-or ted their series of experimentals over five years, that ammoxidation could improve the capacitance of ACs. Their research focused on the capacitance behavior of ammoxidized coal and nanotube carbon materials pur-portedly designed for the electrical industry ( Jurewicz et al., 2006, 2004 and 2002).

Shafeeyan et al. (2010) has provided the precise des-crip tion SNFs plausibly found on activated carbons, espe-cially after amination as depicted in Fig. 2. The research team saw the need to distinguish pyridine and pyridines from each other, mainly when the study of CO2 adsorp-tion is concerned due to the varied affinity for the gas mole cules.

Chemical activation of carbon-based materials with a strong alkali such as concentrated KOH solution has been investigated and reported in recent works (Labus et al., 2014; Mitome et al., 2013). Identical methods that involve soaking and mixing the solid adsorbent with KOH solu-tion, then sintering at a very high temperature under an inert atmosphere were used in all the reports. Usually, due to the increased instability of the carbon surface and the possible presence of highly reactive K2O, the treated carbons are then transferred into a concentrated acid

solution (usually HCl) for neutralization. Some possible inside reactions are presented in equations (1) to (5). Results showed that improved microporosity achieved does enhance CO2 capacity capture of prepared adsor-bents.

6KOH + 2C → 2K + 2K2CO3 + 3H2 (1)

K2CO3 + C → K2O + 2CO (2)

K2O + 2C → 2K + 2CO (3)

K2CO3 → K2O + CO2 (4)

2K + CO2 → K2O + CO (5)

Before 2014, the reported CO2 capture selectivity at indoor levels by modified adsorbents did not show impressive achievement despite of several efforts for the adsorption of IAQ CO2 by AC. Even pre-oxidation treat-ment was later incorporated to populate the AC surface before the displacement reaction induced by the amine radicals. However, not until Adelodun et al. (2014b) ado p- ted the alkalinization method was the target selecti vity met. Later, the KOH treatment concept was adopted for basifying AC surface through alkylation, in which the thermally doped KOH was left on the AC by amine-sta-bilization (Adelodun et al., 2014b). It was the first time that sintered KOH could retain on a carbonaceous sur-face under air atmosphere without any burn-off due to oxidation. In their report, Adelodun et al. (2014b) were able to match the maximum adsorption capacity of modi- fied AC (qmax

(100%-CO2)) with its indoor (0.3%-CO2) selective adsorption capacity of 2.23 and 2.20 mmol/g, respectively.

Fig. 2. Detailed expression of nitrogen-containing complexes possibly found on activated carbons (Shefeeyan et al., 2010).

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4. ACTIVATED CARBON FIBER (ACF)

Generally activated carbon fibers (ACFs) used for CO2 adsorption are prepared from polymeric materials such as polyacrylonitrile (PAN). The systematic processes involved oxidation, activation and amination, focusing on the impregnating SNFs (Adelodun and Jo, 2013). Similar to the basification of AC, the specific surface area and pore volume of ACF were reduced after chemical ami- nation. This is attributed to the presence of more SNFs on the external surface and inner pore walls. Amination is the substitution of a hydrogen bound to a carbon with an amino group.

In our previous studies, the ratio of micropores to the total volume ranged between 0.85 and 0.91, with an aver-age pore width of 1.6 nm. Additionally, the induced SNFs such as imine, pyridine, and pyrrole present favorable affinity with CO2. The aminated ACF exhibited enhan-ced low-level (3,000 ppm) CO2 adsorption from 0.40

mmol/g at room temperature. The selective separation of CO2 against dry air (O2 & N2) also increased from 1.00 to 4.66 by amination.

4. 1 Alkalization of ACFWhile amination is a dry-phase process where ther-

mally decomposed ammonia gas impregnate basic N- groups on the carbon surface, in this study, alkalization is the wet-phase alternative of concentrated solutions of alkali to incorporate alkali or alkali earth metals on the adsorbent. To optimize the advantage of fibrous adsor-bents, activated carbon nanofibers (ACNFs) were fabri-cated via electrospinning, using polyacylonitrile (PAN) as a precursor (Kim et al., 2017). The spun fibers were 390 to 580 nm in thickness, with a surface area in the range of 27.3 to 300 m2/g. The surface structure was improved by programmed thermal activation at 800°C in a CO2 atmosphere, resulting in enhancement of specific surface area up to 542 m2/g. It was also found that the nitrogen-groups including pyrrole and pyridine were created dur-ing the activation, which in turn facilitated the selective adsorption of CO2. The final adsorption capacity was 2.74 mmol/g for pure CO2 flow and 0.74 mmol/g for 3,000 ppm.

Further, the fabrication of doped-ACFs from mela-mine-blended polyacrylonitrile (MACNFs) improved their IAQ-CO2 removal ( Jeong et al., 2019). Here, mela-mine was used as an alkali dopant for basic SNFs. Upon final CO2 activation, the specific surface area and micro-

porosity were enhanced. As per chemical properties, the surface basicity was improved through the significant tethering of pyridine, allowing for preferential adsorption of CO2 over N2. The optimum melamine doping condi-tion achieved the high CO2 adsorption capacity of 3.15

mmol/g. Another attempt to tether SNFs on activated carbon

nanofibers (ANF) was made using tetraethylenepenta-mine (TEPA) solution (Wang et al., 2020) Further impre- gnation of TEPA was achieved with preliminary oxida-tion of the nanofibers with 70% HNO3. The effects of HNO3 and TEPA treatments on the modified ANFs were found that although TEPA impregnation reduced the specific surface area and pore volume of the ANFs from 673.7 and 15.61 to 278.8 m2/g and 0.28 cm3/g, respec-tively, the HNO3 pre-oxidation increased the number of carboxylic groups on the ANF. Upon TEPA loading, pyri-dinic nitrogen was formed, which was further enhanced by pre-oxidation. The surface treatment cumulatively inc-reased the amine content from 5.81% to 13.31%. The adsorbent’s final adsorption capacity toward 3,000 ppm and 100% CO2 levels improved from 0.20 and 1.89 to 0.33 and 2.96 mmol/g, respectively.

4. 2 Comparison between AC and ACFBased on literature review on the properties of AC and

ACF with respect to CO2 adsorption, the respective adv-an tages and disadvantages of the adsorbents are summa-rized in Table 2.

5. EVALUATION OF ADSORPTION CAPACITY

5. 1 Initial Adsorption PerformancePorous activated carbons are very effective for gas

adsorption as driven by the sorbents’ pore structure and surface chemistry. Since the adsorbents are used for tar-get harmful gases in the ambient condition, selectivity is an imperative property that ACs and ACFs should pos-sess. For this reason, their capabilities for a lean gas treat-ment prioritized over their adsorption capacity for pure CO2, as required for CO2 storage and sequestration

(Adelodun and Jo, 2013). The present review investiga-ted mainly selective capture of CO2 in terms of chemical or physical modification of carbonaceous adsorbents.

The adsorption capacity toward 3,000 ppm CO2 (in

binary mixture with N2) was fed to the bed filled with

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adsorbents. Fig. 3 shows a typical lab-scale adsorption set-up for the evaluation (Adelodun et al., 2015; Lee and Jo, 2009). The SENSEair detector was equipped with a non-dispersive infra-red (NDIR) sensor for a continuous sampling. The amount of adsorbed CO2

(mmol/g) was evaluated using Equation (1):

11

Table 2. Comparison of the advantages and disadvantages of AC and ACF 282

S/N Property Activated carbon Activated carbon fiber

1.

2.

3.

4.

5.

6.

7.

8.

9.

10

Adsorption capacity

Adsorption selectivity

Specific surface area

Porosity

Durability

Regeneration

Moisture resistance

CO2 concentration

Fabrication

Surface modification

Slightly higher

Significantly superior

Lower

Hierarchical

Pellets are durable

Requires high temperature or

pressure

Higher, but trapped moisture is

refractory to desorb

Better at higher partial pressures

Numerous starting materials;

process is more tedious

Less homogenous

Slightly lower

Inferior

Higher

Uniform & super-microporous

Depends on precursor

Easier to regenerate

Lower, but easier to desorb

trapped moisture

Better at low partial pressures

Fewer starting materials;

process is less tedious

More homogenous

283

284

5. Evaluation of adsorption capacity 285

5.1 Initial adsorption performance 286

Porous activated carbons are very effective for gas adsorption as driven by the sorbents' 287

pore structure and surface chemistry. Since the adsorbents are used for target harmful gases in 288

the ambient condition, selectivity is an imperative property that ACs and ACFs should possess. 289

For this reason, their capabilities for a lean gas treatment prioritized over their adsorption 290

capacity for pure CO2, as required for CO2 storage and sequestration (Adelodun and Jo, 2013). 291

The present review investigated mainly selective capture of CO2 in terms of chemical or 292

physical modification of carbonaceous adsorbents. 293

The adsorption capacity toward 3,000 ppm CO2 (in binary mixture with N2) was fed to the 294

bed filled with adsorbents. Fig. 3 shows a typical lab-scale adsorption set-up for the evaluation 295

(Adelodun et al., 2015; Lee and Jo, 2009). The SENSEair detector was equipped with a non-296

dispersive infra-red (NDIR) sensor for a continuous sampling. The amount of adsorbed CO2 297

(mmol/g) was evaluated using Equation (1): 298

299

𝑞𝑞 = � ��� [� �1 − ��

���dt�

� ] (1) 300 (1)

where q = amount of CO2 adsorbed (mmol/g), Q = inflow rate of CO2

(200 mL/min), C0 = CO2 inflow con-centration (3000 ppm (i.e., 0.3%)), Ct = CO2 concentra-tion (ppm) at saturation time, t = CO2 sensor intermit-tent read time (s), and M = mass of test sample (g).

As received ACs have been reported to exhibit 3.27 mmol/g adsorption capacity toward pure CO2 flow (Lim et al., 2013). However, its propensity for low level CO2 such as 0.3%, as frequently found in public indoor spaces, was extremely poor (0.016 mmol/g). To maintain a fresh and safe IAQ with less than 2000 ppm, various modifica-tions on the carbonaceous adsorbents that have been exp-erimented are summarized in Table 3. Focusing on field plausibility, commercial-grade alkaline reagents were used as dopants to improve the surface affinity of ACs and ACFs toward CO2.

Although, surface reformation often reduces the intrin-sic specific surface area of carbonaceous adsorbents, caus-ing a low adsorption tendency for pure CO2, such a modi-

Table 2. Comparison of the advantages and disadvantages of AC and ACF.

S/N Property Activated carbon Activated carbon fiber

123456789

10

Adsorption capacityAdsorption selectivitySpecific surface areaPorosityDurabilityRegenerationMoisture resistanceCO2 concentrationFabricationSurface modification

Slightly higherSignificantly superiorLowerHierarchicalPellets are durableRequires high temperature or pressureHigher, but trapped moisture is refractory to desorbBetter at higher partial pressuresNumerous starting materials; process is more tediousLess homogenous

Slightly lowerInferiorHigherUniform & super-microporousDepends on precursorEasier to regenerateLower, but easier to desorb trapped moistureBetter at low partial pressuresFewer starting materials; process is less tediousMore homogenous

Fig. 3. A schematic representation of lab-scale set-up for low-level CO2 adsorption.

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fication adds bilateral interforce, resulting in high selectiv-ity for low CO2 concentration.

Thus, the adsorption capacities of ACs and ACFs for pure CO2 are generally larger than 1 mmol/g, while those of IAQ-CO2 could be as high as 2.2 mmol/g.

Most surface treatments focus on textural modifica-tions, increasing the proportion of supermicropores or ultramicropores with an appreciable degree of consisten-cy. These micro scale pores are preferable for CO2 mole-cules whose kinetic diameter is approximately 3.3 nm.

5. 2 Regeneration and ReusabilityOne of the demerits of zeolites as a viable CO2 adsor-

bent is their affinity for moisture and high energy req-uirement for regeneration. The latter is due to the predo-minance of covalent bonding in attaching with CO2 dur-ing chemisorption (Lee et al., 2012). However, since car-bon-based adsorbents interact with CO2 based on weaker physical or physical-chemical interactions, the energy requirement for their regeneration is lower, and hence more feasible ( Jeon et al., 2016).

Usually, few adsorption studies attempt to perform regeneration studies after estimating the maximum

adsorption capacity. And, because pure CO2 adsorption studies are more popular than those of low-level adsorp-tions, fewer the regeneration and reusability studies on the latter exist (Krishnamurthy et al., 2019). Notably, Adelodun et al. (2014b) achieved the highest adsorption amount for low-level (3,000 ppm) CO2 through five-cycle regenerations on the optimized sample. Generally, the regeneration of AC and ACF sorbents achieves adsorption capacity more than 90% of the initial cycle, provided the physical properties of the sorbent was not compromised during thermal or pressure swing adsorp-tion refreshment.

6. MODELING OF ADSORPTION CHARACTERISTICS

During adsorption, the free, non-adsorbed gas and the adsorbed gas molecules are in a dynamic equilibrium, influenced by temperature, partial pressure or the adsor-bate’s concentration, and the gas composition. The amount of adsorbate can be expressed using either sur-face concentration (mol/m2), or as surface coverage (θ, in

Table 3. CO2 adsorption capacities achieved using ACs and ACFs.

Basis adsorbent Modification CO2 conc. (%) Ads. capacity (mmol/g) Reference

AC

Ca-impregnation 0.3% 0.31 Lee et al., 2012Impregnation of Li with glycine pure 3.08 Lim et al., 2012 NH3 with KOH 0.3% 0.41 Hong et al., 2013UV-oxidation & NH3 0.3% 0.38 Adelodun et al., 2014aNH3 with KOH 0.3% 2.20 Adelodun and Jo, 2013Hi-tem. activation pure 0.1-0.3 Huang et al., 2015

ACF

TEPA impregnation pure 1.89 Wang et al., 20200.3% 0.33

Melamine doping pure 3.15 Jeong et al., 2019 0.3% 0.47

Acetonitrile & ferrocene pure 1.92 Yu and Chuang, 2017Urea doping pure 2.98 Kim et al., 2017 KOH treatment 0.3% 0.05 Hwang et al., 2016Ammonia treatment @ 700°C pure 0.4 Hwang et al., 2016Modified ACF pure 6.58 Choma et al., 2016Graphite fiber-KOH pure 1.61 Yuan et al., 2016Modified ACF pure 3.1 Díez et al., 2015Modified ACF pure 3.4 Díez et al., 2015KOH treatment Pure 5.68 Lee and Park, 2013ACF pure 5.68 Lee et al., 2013Composite pure 2.26 An et al., 2009CO2 activation 0.3% 0.74

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%) which is defined as the ratio of the number of adsorp-tion sites occupied to the number of adsorption sites available (Buekens et al., 2008).

The equilibrium partition between the free and the adsorbed state depends on temperature and pressure. It also can be described in terms of an empirical function q = f (P, T).

q = f (P) Isotherm at constant T

q = f(T) Isobar at constant P

q = f (P, T) Isostere fixed surface coverage

The adsorption isotherm is the most widely used form for representing equilibrium data, because it is the most ameliorable with experimental data. Moreover, this is the form in which theoretical treatments are usually devel-oped. Gas separation by adsorption is achieved by one of the three mechanisms viz steric, equilibrium or kinetic effect ( Jasra et al., 1991). When the adsorption equilibri-um state is determined at a constant temperature, an adsorption isotherm is obtained with changes in pressure. Adsorption isotherms can be used to estimate the surface area and pore volume in various porosity regimes, assess-ments of the surface chemistry of the adsorbent and fun-damental information on the efficiency of various ACs.

The efficiency of adsorbents is estimated on the basis of its capacity, adsorption rate, mechanical strength, and possibility of regeneration and reusability. The adsorbent capacity is the most crucial parameter and the first often determined (Brdar et al., 2012).

Equilibrium data are fundamental information to design an adsorption process. At adsorption equilibrium, the chemical potential of the solute in the gaseous phase is equal to that in the solid phase. In this perspective, equilibrium relationships, describe how pollutants inter-act with adsorbents. Thus, they are critical for optimizing the adsorption pathways, expressing the surface proper-ties and capacities of adsorbents, and the effective design of the adsorption system (El-Khaiary, 2008).

In general, an adsorption isotherm is an invaluable curve describing the phenomenon governing the reten-tion (or release) or mobility of adsorbate from a fluid-phase to a solid-phase at a constant temperature (Limou-sin et al., 2007). In our previous study (Adelodun et al., 2015), six models of the numerous isotherms available

(Foo and Hamed, 2010) were employed to analyze the adsorption of gaseous CO2: 2-parameter models (Lang-muir (L), Freundlich (F) and Temkin (Te) models) and

3-parameter ones (Sips (S), Toth (T) and Redlich-Peter-son (R) models), as could be seen in Fig. 4.

Concerning the theoretical qmax values, only the Redlich-Peterson model was close to the experimental values. Meanwhile, Sips and Toth models, which have been reported in some other studies to exhibit a high degree of fitness with gas adsorption showed conformity to a reasonable extent (Adelodun et al., 2016). Freund-lich is the most reliable two-parameter model to describe the adsorption of CO2 on either pristine or modified AC, while Sips and Redlich-Peterson models are better exp-ressions, probably due to the extra parameter which imp-ro ves their flexibility.

Therefore, the adsorption isotherm study confirms the presence of various SNGs (surface nitrogen groups) heterogeneously distributed on the modified AC. The process favors Freundlich and Langmuir isotherm at low or high CO2 concentrations, respectively (Lim et al., 2016).

7. CONCLUSIONS AND RECOMMENDATION

Despite the impressive adsorption capacities exhibited by activated carbons (ACs) and activated carbon fibers

(ACFs), their selectivity toward indoor-level CO2 is poor. For this reason, chemical modification with various organic and inorganic dopants have been used to basify their surfaces, especially in the past two decades. Since the present review focuses on the field available process, high selectivity and capacity materials that are costly and imperfective molecular organic frames (MOFs) or novel mesoporous zeolitic structure were not considered.

Fig. 4. A typical isotherm model by non-linear fitting for pure CO2 flow with a surface aminated activated carbon.

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We found that amination and ammoxidation offer varying degrees of success with improving both the sur-face chemistry and textural properties. In addition, some alkali materials such as glycine, urea, melamine and tetra-ethyleneamine etc. have been explored to modify the adsorbent surface including pore channels. In particular, the selective adsorption for low level CO2 capture could be improved by alkalinization with concentrated KOH.

Another carbonaceous adsorbent, ACF, is also useful in forming various functional groups suitable for CO2 molecules, because it can be designed to meet the speci-fic purposes from the initial spinning step. The isotherm studies for this diluted CO2 capture seemed to result in Freundlich and Langmuir model implying simple gas adsorption.

The feasibility and performance evaluation for the real-life application of established CO2 adsorbents should be aimed from the view of practical environment. Based on the few empirical studies currently available on the CO2 adsorption from indoor spaces, the use of readily avail-able materials such as waste, biomass, etc. for low-cost adsorbent preparation should be propagated to minimize regeneration cost. Process design studies including reac-tors further should continue with a focus on improving Indoor Air Quality (IAQ).

ACKNOWLEDGEMENT

This study was supported by the National Research Foundation of Korea Grant funded by the Korea Gov-ernment (MSIT, MOE) (No. 2019M3E7A1113077), and partially by Basic Research Fund (NRF-2015R1D1 A1A01060182).

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